When posterior interbody fusion is performed (such as TLIF, OLIF or PLIF technique), it is difficult to insert a large enough fusion cage through the posterior access, especially when the disc space is significantly taller anteriorly than posteriorly. Limiting factors include the maximum height and width of the posterior access to the disc space, as well as the risk of injuring the nerve roots or thecal sac that are immediately adjacent to the pathway of access to the disc. It is not uncommon for a posteriorly inserted fusion cage to fit reasonably tightly (appropriately sized) within the posterior portion of the disc space, but to be loose (undersized) with respect to the anterior portion of the disc space. The inability to insert a properly sized fusion cage may prevent the surgeon from obtaining as much lordosis in the fused segment as desired, and may also increase the risk of post-operative migration or retropulsion of the cage. Both situations may result in reduced efficacy of the surgical treatment.
Attempting to insert a larger fusion cage than the anatomical limitations of the posterior access allow (in an attempt to avoid the problems associated with an under-sized cage) may cause a number of complications such as fracture of the vertebral endplates (which may increase the risk of implant subsidence), over-stretching of the nerve roots (which may result in temporary or permanent neurologic complications), “nicking” the nerve root or dura (which may result in CSF leakage or other neurologic complications), or fracturing the implant due to overly aggressive impaction.
Other surgical approaches to the disc space (such as an ALIF or lateral transpsoas approach) help alleviate some or all of the problems identified above, but may have other drawbacks. The most commonly used approach for interbody spinal fusion is the posterior approach, thus the other potential problems with these alternative surgical approaches often outweigh the problem associated with the posterior access.
Some interbody fusion cages are designed to be inserted into the disc space on their sides, then rotated ninety degrees (90°) into the final position once inside the disc space. This allows an implant that is tall but narrow to be inserted with less cephalad-caudal distraction of the anatomy during insertion. However, the tradeoff is that the medial-lateral width of these cages is larger during insertion. Especially in the case of very tall cages (such as a cage with a high degree of lordosis), the anatomy may not accommodate the required cage height when it is turned on its side for insertion. These “insert and rotate” cages are also associated with other potential complications such as inability to rotate the cage into final position (this rotation requires some over-distraction of the disc space, which may not always be desirable or achievable), or fracture of the cage due to the twisting forces required to rotate the cage into position.
Other interbody fusion cages solve the problem by providing for expansion of the implant after it is placed into the disc space. The existing art demonstrates a number of different designs for achieving this, each of which has its own pros and cons.